1. Field of Invention
This invention relates to a method and apparatus for feeding media from a roll, for a calibrated distance, to a work station.
2. Background Information
In certain applications, it is necessary to transport media (like paper, magnetic tape, or inked ribbons, for example) a fixed distance from a supply spool, on which the media is wound into a roll, to a work station. From the work station, the media is transported to a take-up spool on which the media is wound. As the media is used, the diameter of the media on the supply spool gets smaller while the diameter of the media on the take-up spool gets larger. In such a situation, the rotational motion of the supply spool and the take-up spool will vary depending upon how much media there is on each one of these spools. In such a situation, it is not appropriate to move the take-up spool through a fixed rotational angle to move the media a fixed distance at the work station.
There are three general methods of solving the problem presented in the previous paragraph. These methods are:
1. Drive the media;
2. Use a metering roller on the media; or
3. Drive the take-up spool through varying angles according to the spool diameter.
The first method presented above is useful for media which is narrow because wide media does not track well with this method. It is also undesirable to use a pinch roller on printing media because one or both of the rollers in the associated capstan-pinch roller combination becomes contaminated with ink, resulting in operation failure.
The second method presented above places a roller arrangement in the path of the media. The media is deflected around the roller which turns as the media is advanced. This technique is generally adequate; however, it depends upon two factors. They are:
1. No slip between the media and the roller; and
2. The media motion has to be in phase with the drive.
The first limitation is usually the more critical. Very often, the media has a slippery side which contacts the roller; this makes the friction between the media and the roller hard to control; and consequently, feeding the media a fixed distance is difficult to control. The second limitation is more subtle. When driving the media, it is normal to accelerate the media until the media has moved through a certain distance and then decelerate the media. If there is a phase shift between the motor moving the media and the media itself, there will be an error in movement of the media. This is especially prevalent when a slotted cardboard core, on which the media is wound, is used.
The third method mentioned above, driving the take-up spool through varying distances, relies upon a fundamental assumption. That is, if the total volume of media on each of the supply and take-up rolls is known, then the relative turning arcs of each spool can be determined for any diameter of the take-up roll. For example, if the supply and the take-up spools are half full, then both these spools will turn the same amount for an increment of media to be supplied to a work station. By measuring the ratio of the number of steps moved by the take-up spool relative to the supply spool, the diameter of the media on the take-up spool can be calculated. This technique is applicable if, and only if, the following events apply:
1. The take-up tension (at the take-up spool) is the same as the supply tension (at the supply spool);
2. The total volume of the media is known; and
3. There is a direct coupling between the supply spool and the take-up spool.
This method is difficult to achieve because it is difficult to realize the above three events. For example, the tension is usually much higher at the take-up spool that it is at the supply spool. The media volume can change drastically, especially after media breakage. Generally, some sort of tensioning arm is used as an inertia-reducing element; this decouples the supply spool from the take-up spool.